Identification of Compounds that Protect HERG from Block by Proarrhythmic Agents The acquired/drug induced Long QT Syndrome (aLQTS) is a threat to public health and a major stumbling block for drug development. It is most often caused through unintended block of the cardiac repolarizing potassium channel, IKr, encoded by the Human Ether-`-go-go Related Gene (HERG). HERG is susceptible to inhibition by even transient exposure to a diverse array of common therapeutic compounds, including antihistamines, antidepressants, antibiotics, antiemetics, and antiarrhythmics that in turn provokes the life-threatening cardiac arrhythmia torsades de pointes in predisposed individuals. Indeed, several successful drugs (e.g., cisapride, seldane) had to be withdrawn from the market due to arrhythmia development caused by this mechanism. Our previous studies have shown that IKr can be protected from block by known HERG antagonists through the activity of the K+ channel regulator 1 (KCR1) protein which we characterized as an enzyme that functions in the cellular glycosylation pathway. This finding led to our hypothesis that identification of a small molecule that mimics the activity of KCR1 and diminishes IKr block by known HERG inhibitors may lead to the development of a co-drug to protect against the development of the aLQTS. We will use established high throughput methods to: 1. Screen a library of 130,000 compounds for agents that increase the IC50 of the well- characterized HERG inhibitor, dofetilide, using a Tl+ flux assay. Our preliminary data show that the assay is highly suitable for this type of screening. 2. Determine if the isolated hits also affect the IC50s of a range of drugs with divergent chemical structures (moxifloxacin, clozapine, and droperidol) and clinical applications in the Tl+ flux assay. Our findings will be validated through standard patch-clamp assays. The development of a small molecule chemical agent that could be safely co- administered with clinically useful pharmaceuticals to protect susceptible patients from HERG inhibitors and the subsequent development of arrhythmias would improve public health and greatly facilitate the drug discovery process. PUBLIC HEALTH RELEVANCE: Potentially lethal cardiac arrhythmias can be the tragic consequences of treating patients with otherwise useful therapeutic agents for non-cardiac conditions. The most common cause of this unpredictable effect is the unintended block of a cardiac potassium channel, called HERG that is essential for maintaining the balanced electrical activity and rhythmic contraction of the heart. The goal of this proposal is to utilize high throughput screening methods to identify a chemical compound that diminishes unintended HERG block, which could eventually be co-administered and thereby reduce the risk of arrhythmia development. [unreadable] [unreadable] [unreadable]